Apparatus for producing single crystal metallic alloy objects



United States Patent 1 3,542,120 [72] inventor Barry J. Plearoey [56] References Cited Brixhani, Devon, England [21] 764471 3 204 303 9/12 2? h jl PATENTS Filed 0 1, i r all v r 3,283,377 11/1966 Chandley l64/36lX Division of540,114, Feb. 17, 1966, now Pat. 3 342 455 9/1967 F] k t l 164 361UX No. 3,494,709, which is a continuation-incc 6 a part of Ser. No. 459,391, May 27, 1965, Primary ExaminerJ. Spencer Overholser abandoned. Assistant Examiner-John E. Roethel [45] Patented Nov. 24,1970 Attorney-Morgan, Finnegan, Durham & Pine [73] Assignee United Aircraft Corporation East Hartford, Connecticut in corporation of Delaware [34.] APPARATUS FOR PRODUCING SINGLE CRYSTAL METALLIC ALLOY OBJECTS 1t) Claims, 9 Drawing Figs. [52] US. 164/361 [5 1] Int. C 1322c 9/02 [50] Field olSeaI-ch 164/338,

353, 361, 6 0, 127; l48/l.6; 415/177, 216; 4l6/24l 23(lnquired) ing a face-centered cubic crystal structure, said crystal being oriented with its [001] direction less than 20 from the elongated axis of the crystal, said single crystal object having laterally enlarged integral base and generally being in the form of a gas turbine blade or vane.

caew/w/c M040 Patented Nov. 24, 1970 3,542,120

Sheet 1 or 3 Patented Nov. 24, 1910 Sheet. 2 6r 3 (F t mm fB/XML Patented Nov. 24, 1970 3,542,12

Sheet 3 of 3 /IIIIIII APPARATUS FOR PRODUCING SINGLE CRYSTAL METALLIC ALLOY OBJECTS This application is a divisionof my prior application Ser. No. 540,1]4 filed Feb. l7, l966 now US. Pat. No. 3,494,709 which is a continuation in part of my application Ser. No. 459,39l;filed May 27, I965 and now abandoned.

The present invention relates toa novel and improved process and mold for the formation of elongated shaped objects comprising a single crystal oriented in a particularly desirable direction and to an apparatus useful in carrying out the process, as well as to novel and such improved singlecrystal blades and vanes fora gas turbine engine, especially those blades and vanes formed from certain nickel-based alloys.

Thepresent invention has for its object the provision of a novel and improved process for the formation of shaped objects, suchasan elongated blade or vane for a gas turbine engine formed as a single specially oriented crystal. A further object is the provision of a novel and improved apparatus which is especially adapted to the consistent production of such single-crystal shaped objects.

While the apparatus of. the present invention are of wide usefulness in the formation of single crystal objects of relatively complex shape, havingthe crystal axis in a predetermined relation to the shape of the object, the single-crystal objects of l the invention, and especially the blades and vanes suitable for use in a gas turbine engine are most usefully formed from I nickel-basetsuperalloys, especially those alloys which are commercially known as Mar-M 200, and most preferably from Mar-M 200, which is substantially free of both boron and zirconium and with extremely low carbon content. According to the,present invention, the preferred and illustrative apparatus for forming the cast single-crystal objects having a crystalline orientation substantially parallel to the length of the cast object comprises a ceramic shell mold,

usually formed by the lost-wax method" which rests upon a thermally conductive surface, preferably adapted to be water cooled, which shell moldis adapted to be radiantly heated by aninductively heated graphite susceptor, so that it may be brought to substantially the temperature of the melting point of the alloy to be cast, or slightly higher initial temperature at the upper part of the mold, while the water-cooled support remains substantially below the melting point, to facilitate solidification of the alloy to be cast.

The mold comprises an enlarged base cavity, which is connected to one or more shaped mold cavities by inclined passageways which are greatly restricted with respect to both When molten metal of an alloy which crystallizes as a facecentered cubic crystal is poured into the heated shell mold, as they are supported on a water-cooled supporting member, the

The upper end of the restricted passageway is completely laterally offset from the lower end so that changes of direction are required as the metal solidifies beginning in the base cavity l andeventually in the mold cavity proper. metal in the lower enlarged'central portion of the shell mold, on cooling, crystallizes and grows more rapidly along the axis, the lower portion of the constricted portion of the shell mold becomes filled 'with oriented columnar crystalline alloy which tends to grow sidewise and upwardly, and gradually induces the molten alloy fillingthe shaped portion of the molds to solidify as a single oriented crystal having its direction substantially coinciding with the elongated or principal stress axis of the object being cast.

The casting operation is preferably carried out in vacuum, or in an inert atmosphere, preferably argon, although for less demanding uses, the casting operationmay be carried out in arr.

After casting, the cast objects may be heat treated to improve their mechanical and physical properties, before or after which the lower portion of the casting and the sprue may be cut away, and any necessary machining may be done on the cast objects.

While the mold of the present invention is of wide usefulness with many different metals, alloys and other substances which crystallize on cooling, the blades and vanes to be used in a gas turbine, according to the present invention are alloys having compositions falling within the following weight percent ranges:

Percent Chromium 2-25 Cobalt 4-30 Aluminum Up to 9 Titanium Up to 6.0 Molybdenum and tungsten 2-14 Carbon Up to 0.5 Boron Up to 0.1 Zirconium Up to 0.2

the balance of the alloy being essentially nickel in an amount of at least 35 percent.

Alloys which are especially adapted for use with the present invention and are preferred have the following elements in the weight percentage ranges set forth below, it being understood that copper, manganese, sulfur, and silicon are generally considered the impurities.

PWA Alloy Numbers Chromium Cobalt.

Tungsten. Molybdenun Columbium Aluminum l Titanium- Tantalum- Vanadium. 0. 7-1. 2 Boron 0. 01-0. 04 0. 005-0. 15 0. 01-0. 02 0. 025-0. 035 0. 01-0. 02 Zirc0nium.. l 0. 06 0. 05-0. 12 0. 03-0. 09 l 0. 06 0. 03-0. 08 on 0.5 3.0 1.0 4.0 1.5 l 0. 2 0 15-0. 20 0. 03-0. 1 0. 09-0. 17 0.5 0.1 0.10 1.0 0.2 0.15 0.20 0. 015 l 0. 015 1 0.015 1 0. 015 0.2 1.0 0.2 0.2 0.2 Nickel a Balance Balance Balance Balance Balance 1 Maximum. 2 Plus Ta. 3 Essentially.

Even better results are obtained where the quantities of boron and zirconium are further reduced, and most preferably the boron is present in a maximum quantity of less than 0.001 percent by weight, while the zirconium has a maximum of 0.0l percent by weight, Alloy PWA No. 659 is especially benefited by reduction of the boron and zirconium content to these very low limits.

While the usual minor amounts of boron and zirconium are highly advantageous in objects made of PWA Alloy No. 659 Mar-M 200 which have a conventional heterogeneous, equiaxed crystalline structure, the presence of boron and zirconium in a single crystalline structure is distinctly disadvantageous in several respects, such as:

The boron and zirconium lower the melting point of the alloy, and thus lead to a lowering of the creep resistance of the part made from the alloy.

The boron and zirconium content of the alloy is concentrated in small areas in the dendritic structure of the crystal, thus producing weakening discontinuities in the structure with a consequent impairment of all of the various properties of the single crystal blade or vane. In the utilization of the present invention, where the blades and vanes are in the form of single crystals, the presence of minute dispersed concentrations of boron or zirconium within the crystal is a distinct disadvantage and is to be eliminated so far as commercially feasible.

Due to the fact that all commercial raw materials are impure, and that it is commercially impractical to obtain a pure raw material, such relatively pure materials are used, care being taken that they do not introduce excessive amounts of impurities. In the use of the present invention, for optimum results, care is taken that the raw materials are substantially free of boron and zirconium, and that the raw materials are melted in proper crucibles which introduce no excessive amounts of impurities. Magnesia crucibles are ordinarily avoided as they are a frequent source of boron contamination. Zirconia crucibles are a frequent source of zirconium. Alumina and aluminum silicate crucibles are preferred as they may be obtained substantially free of boron and zirconium contaminating constituents.

All of the nickel base, heat and corrosion resistant alloys, such as PWA Nos. 101 IA, 655, 658, 659 and 689 are beneficial in the single crystal condition by reduction of the boron and zirconium content, as specified above.

The limit of 0.001 percent for boron and 0.01 percent for zirconium is based upon the type of analytical procedure used. The presence of boron and zirconium in the alloy can be confirmed by analytical methods, but the analytical procedure necessary to provide quantitative values below the values of 0.001 percent boron and 0.01 percent zirconium was not attempted in this instance. For this reason, an accurate, precise maximum content of these elements has not been established, except as herein specified.

The apparatus of the invention is useful in connection with the production of single crystal cast objects from a wide variety of alloys forming face-centered crystals.

According to the present invention, the resulting cast objects are almost always in the form of single crystals properly oriented with respect to the stress axis of the cast object.

Of the drawings:

FIG. 1 is a schematic vertical sectional view through a mold of the present invention.

FIG. 2 is a cross-sectional view taken on the line 2-2 of FIG.

FIG. 3 is a similar schematic vertical section showing a modified embodiment of a mold in accordance with the present invention.

FIG. 4 is a cross-sectional view taken on the line 4-4 of FIG.

FIG. 5 is a schematic vertical section showing a further modification of the mold of the present invention, especially adapted for the casting of a plurality of elongated objects each formed ofa single crystal ofmetal or other material.

FIG. 6 is a vertical section showing a casting apparatus for use in accordance with the present invention, together with legends showing the several materials and the states of the molded object.

FIG. 7 is a perspective view of an illustrative form of rotor blade for a gas turbine produced in accordance with the present invention.

FIG. 8 is a cross-sectional view showing a modified embodiment of a turbine blade produced in accordance with the present invention.

FIG. 9 is a perspective view of an illustrative form of a vane member for a gas turbine produced in accordance with the present invention.

Referring now in detail to the present preferred and illustrative embodiments of the present invention, a simple form of mold for carrying out the process of the present invention is shown in FIGS. I and 2, in which there is provided a shell mold having an interior shape appropriate to the object to be molded. This mold comprises a relatively thin-walled shell which has preferably been formed by shell molding technique for use in the lost-wax method of casting, and is to be used in a relatively high vacuum, less preferably in an inert atmosphere of argon or helium, or sometimes in an atmosphere of air.

The mold 20 is formed to rest on a relatively cool, heat conductive, and preferably water-cooled block 22, which is conveniently made ofa relatively thick piece of copper or copper alloy. The block during the casting process is maintained at a temperature considerably below the solidification temperature of the alloy or other material to be cast.

The lower portion of the mold 20 comprises a relatively wide cavity 24 which communicates with and upwardly inclined restricted passageway 26 connecting the base cavity 24 with the bottom of the mold cavity proper 28. The passageway 26 may be of circular cross section, as shown in FIG. 2, or may be otherwise shaped, but is nonlinear and has a relatively small cross section compared with the cross section of the lower portion, and is preferably upwardly inclined to communicate with the mold cavity 28. This restricted passageway terminates in a horizontal planar junction with the bottom of the mold cavity proper 28, as shown in FIG. I, and also in FIGS. 3, 5 and 6.

The mold is preferably formed of ceramic material from a conventional slurry of alumina or other high melting point refractory material, in accordance with standard shell-molding techniques.

FIGS. 3 and 4 illustrate an improved and preferably form of molding apparatus in accordance with the present invention. In this form, the shell mold 30 is formed to provide a base cavity 32 which communicates with a laterally extending and preferably upwardly inclined nonlinear passageway 34 which leads to a second restricted laterally extending passage 36, which preferably extends in a different direction, and communicates with the mold portion proper 38. Mold 30 is open at the top to receive the molten metal from which the object is to be molded, and rests upon a relatively cool and preferably water-cooled copper block 22 which establishes a temperature gradient within the molten metal filling the mold, so that solidification of the alloy within the mold begins at the bottom of the mold.

As shown in FIG. 4, the restricted passageway 34 is preferably a relatively narrow slot, and the portion 36 is similarly shaped, to assist in insuring that the solidified metal within the mold portion proper 38 is in the form of a single crystal, the crystal axis extending lengthwise of the mold portion 38, that is, in a substantially vertical direction.

FIG. 5 illustrates a form of molding apparatus in which a plurality of mold cavities 50 are connected with a single base cavity 52 resting on a copper cooling block 22. Each of the cavities 50 is connected with the base cavity by means of a restricted, laterally and upwardly extending passageway 56. At their upper ends, mold cavities 50 are connected with a central mouth 58 through which the molten metal is introduced to fill the several parts of the molding apparatus.

FIG. 6 of the drawings illustrates schematically and in a more complete manner a molding apparatus according to the present invention for carrying out the process of the present invention. The entire apparatus shown in FIG. 6 is preferably enclosed within a vacuum chamber (not shown) or within a chamber which may be filled with argon or other inert gas. The mold portion 60 provides an enlarged base cavity 61, above which is the portion 62 providing an upwardly and laterally extending restricted passageway 63 communicating with the mold cavity 65 formed by the shell 64. Above the mold cavity 65 is the pouring mouth 66 formed by the upper most portion of the shell, and within which the sprue 67 forms.

Surrounding the shell mold are the means for heating the mold to the desired temperature for casting. Preferably, the shell mold is surrounded by a graphite susceptor 70, and this in turn is surrounded by an induction coil 72 supplied with high frequency electric current as is usual in a high frequency induction furnace. Prior to casting, the shell mold is seated on the cooling block 22, the chamber is evacuated or filled with inert gas and the coil 72 is supplied with current to heat the shell mold to the desired temperature for casting. When the desired temperature has been attained, the molten metal,

heated to the proper temperature for casting, is poured into the mold mouth 66 to fill the mold, the copper chill block being maintained relatively cool so as to establish a temperature gradient within the molten metal filling the mold as the metal solidifies. Power is shut off from the coil 72, and the assembly is allowed to cool.

After completion of the process using the present invention, the shell mold and cast metal are removed from the furnace, and the shell mold is broken away from the cast object, after which the surplus metal is machined away to provide the cast object, after which the surplus metal is machined away to provide the cast blade or vane member formed by the mold cavity The metal within the base cavity 61, when the metal is a face-centered cubic crystalline alloy, has a controlled columnar crystalline structure, with the crystals extending upwardly within the base portion and'into the restricted passageway 63. With the restricted passageway 63, the solidified metal becomes a single crystal which fillsthe mold cavity 65, the crystal axis extending substantially vertically along the length of the blade or vane member. This single crystal structure extends into the mouth 66 of the shell mold, and the sprue portion 67 generally exhibits an uncontrolled polycrystalline growth.

FIG. 7 of the drawings illustrates a rotor blade for use in a gas turbine, which blade is of conventional shape, but is differentiated from the rotor blades of the prior art by being formed of a single crystal of a face-centered cubic crystalline alloy, the single crystal having a orientation with respect to the elongated axis of the blade member. Exact coincidence between the direction of crystallization of the single crystal forming the blade member and the longitudinal axis of the blade member is not essential and as much as a 20 deviation between the crystal direction and the longitudinal axis is acceptable, it being understood that the closer the crystal direction and the longitudinal axis coincide, the more fully the principal objects of the present invention are achieved.

As shown in FIG. 7, the rotor blade member comprises a root member 80, a shroud portion 82 and an intermediate airfoil portion 84, all portions of which are formed as a single crystal of a face-centered cubic crystalline alloy, having the composition of the broad range of nickel base alloys set forth above, and most preferably having a composition set forth with respect to the alloy designated as PWA 659 Mar-M200.

Airfoil gas turbine members which are to be subjected to in ternal cooling during operation may be provided with an internal passage or passages through which a cooling fluid is circulated during operation of the turbine. Such a blade is shown in section in FIG. 8, the blade otherwise being shaped in accordance with that shown in FIG. 7. In FIG. 8, the airfoil section, root and shroud portions are formed with a smooth interl nal passage or passages, and as shown the passage 86 is formed in the blade. Like the blade of FIG. 7, the blade of FIG. 8 having a longitudinally extending interior passage 86 is formed as a single crystal of a face-centered cubic crystalline alloy having its orientation substantially coinciding with the longitudinal axis of the blade member.

FIG. 9 of the drawings illustrates a conventional form of vane member 88 for use as an airfoil member in a gas turbine, and which is formed as a single crystal of a face-centered cubic crystalline alloy in which the direction substantially coincides with the principal longitudinal axis of the vane member.

The process of the present invention is illustratively described especially with respect to the apparatus shown in FIG. 6 of the drawings:

A shell mold having a mold cavity 65 of the desired shape, an enlarged base cavity 6!, 'a laterally and upwardly directed restricted passageway communicating between the base cavity 61 and the mold cavity 65 and provided at its top with an enlarged, upwardly extending mouth 66 is firmly seated on a copper chill block 22 within a vacuum induction furnace. The

shell mold is preheated by current supplied to the induction coil 72, thereby heating the susceptor element and the shell mold itself. The shell mold at its lower end is maintained at a lower temperature by means of the copper chill block 22 which is cooled by means of water circulating in a lower portion of the block 22.

The shell mold is preferably heated to a temperature of about 2600F. for the casting of PWA 659, and the temperature of the upper face of the chill block 22 is preferably maintained at a temperature of not more than 200F.

The interior of the furnace is either evacuated to a pressure of 10- mm.(l-Ig) or less, or is purged and filled with an inert gas, preferably argon.

A suitable quantity of the alloy to be cast, such as PWA 659, is then melted within the furnace by high frequency inductive heating, and when the molten alloy has been heated to a temperature above its melting point, preferably to a temperature of about 2600F., the alloy is poured into the mold so as to completely fill the mold.

The molten alloy immediately begins to solidify at its lower portion within the base cavity 61 where the molten alloy is in contact with the cool, chill plate 22. Initially there is formed a very thin layer of uncontrolled polycrystalline solidified alloy on the surface of the chill block 22. These uncontrolled crystals having a haphazard orientation give way to the more rapid upward growth of thecrystals so that in the upper portion of the base cavity 61 the crystals are substantially all of orientation. As the crystal growth proceeds upwardly through the cooling mass of metal in the mold, a few of the upwardly growing crystals having an orientation enter the restricted laterally and upwardly directed passageway 63 and one crystal continues to grow laterally and then upwardly into the mold cavity 65, and the growth in the major portion of the restricted passageway 63 and completely in the mold cavity 64 is a single crystal of the face-centered cubic crystalline alloy.

During the solidification of the alloy heat is continually drawn away by the water-cooled copper chill block 22 so that a temperature gradient is always maintained between the bottom portion and the upper portion of the metal within the shell mold.

After the casting and solidification of the elongated object within the shell mold has been completed and has cooled to a moderate temperature at which the single crystal cast part is no longer subject to deleterious action by exposure to air, the chamber may be opened to break the vacuum or to allow air to enter the furnace chamber, and the shell mold and its enclosed cast part may be removed from the furnace. When the shell mold and part have cooled, the shell mold may be broken away from the cast part, and the cast part is then ready for machining to accurately finish its root and shroud portions, and for any finishing which may be required on the airfoil section, although such machining of the airfoil section is generally not required.

Test specimens of face-centered cubic crystalline alloy parts of blades and vanes produced in accordance with the present invention exhibit surprisingly superior properties compared with uncontrolled polycrystalline and directionally solidified parts of the same alloys, as fully described in my copending application Ser. No. 540,114 filed Feb. 17, 1966 now U.S. Pat No. 3,494,709.

I claim:

1. A molding apparatus for casting a single crystal structure 2. A molding apparatus according to claim 1 in which the passageway has a longer vertical than lateral dimension.

3. A molding apparatus for casting a single crystal structure of complex shape, the mold comprising an enlarged base portion on a chilled plate, a plurality of mold portions for the finished parts; upwardly inclined restricted passageways connecting each mold portion with the base pcfi'tion, and means for heating the mold portions to maintain a temperature gradient from the chilled plate to the upper portions of the mold.

4. A molding apparatus according to claim 1 in which the restricted passageway terminates in a horizontal planar junction with the bottom of the mold cavity proper.

5. A molding apparatus including a mold for casting a single crystal structure of complex shape, the mold comprising an enlarged base portion on a chilled plate, a mold portion for the finished part, an elongated, upwardly inclined, restricted passageway connecting the base portion with the mold portion terminating in a horizontal planar junction with the bottom of the mold cavity proper, and means for heating the mold to maintain a temperature gradient from the chilled plate to the heated upper part of the mold portion.

6. A molding apparatus including a mold for casting a single crystal structure of complex shape, the mold comprising an enlarged base portion on a continuously chilled plate, a plurality of mold portions for the finished parts; upwardly inclined restricted passageways connecting each mold portion with the base portion terminating at their upper ends at the bottoms of the mold portions, and means for heating the mold portions to maintain a temperature gradient from above the melting point of the alloy at the heated upper portions of the mold to the chilled plate below the bottoms of the mold portions.

7. A molding apparatus including a mold for casting a single crystal structure of complex shape, the mold comprising an enlarged base portion on a continuously chilled plate, a mold portion for the finished part, a restricted passageway connecting the base portion with the mold portion and having its upper end laterally offset with respect to the bottom end of the passageway and means for heating the mold to maintain a temperature gradient from above the melting point of the alloy at the mold portion to the chilled plate.

8. A molding apparatus including a mold for casting a crystal structure of complex shape, the mold comprising an enlarged base portion on a chilled plate, a plurality of mold portions for the finished parts, restricted upwardly inclined passageways, each of which is longer than its width, and having its upper end laterally offset with respect to the bottom of the passageway and connecting each mold portion with the base portion, and means for heating all of the mold portions to maintain a temperature gradient from the chilled plate to the heated upper portions of the mold.

9. A molding apparatus including a mold for casting a single crystal structure of complex shape, the mold comprising an enlarged base portion on a continuously chilled plate, a mold portion for the finished part, an upwardly inclined, elongated restricted passageway which is longer than its width connecting the base portion with the mold portion and having its upper end laterally offset with respect to the bottom end of the passageway and terminating in a horizontal planar junction with the bottom of the mold cavity proper and means for heating the mold at least to substantially the temperature of the melting point of the alloy to be cast to maintain a temperature gradient from the chilled plate to the heated upper part of the mold portion.

10. A molding apparatus as recited in claim 1 in which said restricted passageway has one portion extending upwardly in a first direction and another portion extending in a direction different from said first direction. 

